shader_runner: add the ability to enable/disable GL_PROGRAM_POINT_SIZE
gs: Test all compatibility-only inputs/outputs except gl_TexCoordIn.

gl_TexCoordIn will need more aggressive testing since it's a 2D array.
!UPSTREAM: gs: Test proper behaviour of gl_TexCoordIn.

These tests are based on the existing tests for gl_ClipDistanceIn,
which behaves similarly.

TODO:
- Test aoa stuff on nVidia
!UPSTREAM: GL 3.1: test that core features are properly excluded from old GLSL versions.

TODO:
- Do I need a better commit message?
- Test on nVidia
!UPSTREAM: gs: Test all core inputs except gl_PrimitiveIDIn.

gl_PrimitiveIDIn will need more aggressive testing, so that will be
addressed in a separate patch.

TODO:
- Add core outputs (gl_ClipDistance and gl_PointSize already done)
gs: Test gl_PrimitiveIDIn